1. Field of the Invention
The present invention relates to an antenna device and a transformer between a waveguide and a transmission-line.
2. Description of the Related Art
High-frequency waves experience a large loss through a waveguide relative to its length because they have a short wavelength. Therefore, an antenna element and a high-frequency circuit are better suited to be close to each other in order to decreases the loss in the waveguide. Size of the antenna element becomes smaller with shortening of the wavelength. It is difficult to make such a small antenna element with high precision. Moreover, not only an antenna element but also a feed circuit and the high-frequency circuit preferably have small sizes in order to minimize size of a radio apparatus.
One technique to minimize the size of the antenna element, the feed circuit and the high-frequency circuit is disclosed in JP-A 2005-204344 (KOKAI). In this reference, the antenna element (which is a waveguide array including several waveguides), the feed circuit and the high-frequency circuit are integrated in a slot array antenna device. Each waveguide is formed by depositing an electric conductor on the surface of a dielectric block. The waveguide array is formed by combining several waveguides and then forming slots on the waveguide array by photolithography. Moreover, the feed circuit and the high-frequency circuit are stacked on the waveguide array.
In the above-mentioned reference, because the waveguide array and the high-frequency circuit are set to be close to each other by stacking the antenna elements, the feed circuit and the high-frequency circuit, a slot antenna device is small and lightweight. Moreover, photolithography can realize higher manufacturing precision as compared with machining.
However, the antenna device becomes thicker because there are three layers of the antenna element, the feed circuit and the high-frequency circuit. Moreover, the antenna device needs a balun in order to convert a differential signal to a single-ended signal. Therefore, the structure of the antenna device becomes complex.